As is known, there are various piezoelectric ceramic compositions such as, for example, those containing PbTiO.sub.3 as a major component, or those containing Pb(Ti, Zr)O.sub.3 as a major component, or their substitutional solid solutions containing Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3 or Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 as a second or third component. Among them, the compositions of the binary or ternary system comprising Pb(Ti, Zr)O.sub.3 as a major component are improved in piezoelectric characteristics and electrical characteristics by incorporating various additives, and have been used as a material for pyroelectric elements, piezoelectric vibrators, resonators, ceramic filters, surface acoustic wave filters and the like. However, the piezoelectric ceramics with these compositions have essentially a high dielectric constant, so that the devices for high frequencies such as resonators, ceramic filters and surface acoustic wave filters possess extremely low impedance, resulting in the difficulty in matching impedance of the devices with impedance of external circuits.
In contrast therewith, the PbTiO.sub.3 ceramics with minor substitution of Pb sites have low dielectric constant. However, they have hardly been put into practical applications because of their poor temperature characteristics.
It is generally known that bulk waves or surface acoustic waves which propagates in or on ceramics being a single phase has a tendency that its propagating rate slows down monotonously with rise of temperature. This reflects on a phenomenon that the ceramics is softened with the rise of its temperature. Considering the influence of this phenomenon upon characteristics of the piezoelectric ceramics, it leads to decrease of the resonant frequency corresponding to the rate of bulk waves or surface acoustic waves with the rise of temperature. Accordingly, in order to produce piezoelectric ceramics of the PbTiO.sub.3 system having excellent temperature characteristics without deterioration of other dielectric and piezoelectric characteristics which are enough for practical applications, it is necessary to take a proper step such that the resonant frequency is not considerably decreased with rise of temperature or, conversely, the resonant frequency is increased with rise of temperature. In other words, to keep the propagating rate of bulk waves or surface acoustic waves constant, the ceramics must be so improved that the ceramics do not abruptly soften with the rise of temperature, or that the ceramics harden with the rise of temperature.